Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.
Methods for the chemical synthesis of butanols are known. For example, 1-butanol may be produced using the Oxo process, the Reppe process, or the hydrogenation of crotonaldehyde (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). 2-Butanol may be produced using n-butene hydration (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719). Additionally, isobutanol may be produced using Oxo synthesis, catalytic hydrogenation of carbon monoxide (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCHVerlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719) or Guerbet condensation of methanol with n-propanol (Carlini et al., J. Molec. Catal. A: Chem. 220:215-220 (2004)). These processes use starting materials derived from petrochemicals, are generally expensive, and are not environmentally friendly.
Methods of producing butanol by fermentation are also known, where the most popular process produces a mixture of acetone, 1-butanol and ethanol and is referred to as the ABE processes (Blaschek et al., U.S. Pat. No. 6,358,717). Acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum is one of the oldest known industrial fermentations, and the pathways and genes responsible for the production of these solvents have been reported (Girbal et al., Trends in Biotechnology 16:11-16 (1998)). Additionally, recombinant microbial production hosts expressing a 1-butanol biosynthetic pathway (Donaldson et al., copending and commonly owned U.S. Patent Application Publication No. US20080182308A1), a 2-butanol biosynthetic pathway (Donaldson et al., copending and commonly owned U.S. Patent Application Publication Nos. US20070259410A1 and US 20070292927A1), and an isobutanol biosynthetic pathway (Maggio-Hall et al., copending and commonly owned U.S. Patent Publication No. US 20070092957) have been described. However, biological production of butanols is believed to be limited by butanol toxicity to the host microorganism used in the fermentation.
Bacteria of the genus Clostridium naturally produce butanol and have some natural tolerance to butanol. Strains of Clostridium that have increased tolerance to 1-butanol have been isolated by chemical mutagenesis (Jain et al. U.S. Pat. No. 5,192,673; and Blaschek et al. U.S. Pat. No. 6,358,717), overexpression of certain classes of genes such as those that express stress response proteins (Papoutsakis et al. U.S. Pat. No. 6,960,465; and Tomas et al., Appl. Environ. Microbiol. 69(8):4951-4965 (2003)), and by serial enrichment (Quratulain et al., Folia Microbiologica (Prague) 40(5):467-471 (1995); and Soucaille et al., Current Microbiology 14(5):295-299 (1987)). Overexpression in Clostridium of the endogenous gene encoding cyclopropane fatty acid synthase increased the cyclopropane fatty acid content of early log phase cells and initial butanol resistance (Zhao et al. (2003) Appl. and Environ. Microbiology 69:2831-2841).
Desmond et al. (Appl. Environ. Microbiol. 70(10):5929-5936 (2004)) report that overexpression of GroESL, two stress responsive proteins, in Lactococcus lactis and Lactobacillus paracasei produced strains that were able to grow in the presence of 0.5% volume/volume (v/v) [0.4% weight/volume (w/v)] 1-butanol. Additionally, the isolation of 1-butanol tolerant strains from estuary sediment (Sardessai et al., Current Science 82(6):622-623 (2002)) and from activated sludge (Bieszkiewicz et al., Acta Microbiologica Polonica 36(3):259-265 (1987)) has been described. However, for most bacteria described in the art, particularly those that do not naturally produce butanol, growth is highly inhibited at low concentrations of 1-butanol.
There is a need, therefore, for bacterial host strains which do not naturally produce butanol but can be engineered to express a butanol biosynthetic pathway, to be more tolerant to these chemicals. In addition there is a need for methods of producing butanols using bacterial host strains engineered for butanol production that are more tolerant to these chemicals.